1. Field of the Invention
This invention relates to an output amplifier providing a high output voltage swing for fast pulse amplification in pulse generators, comprising a first bipolar transistor linear amplifier stage receiving an internal pulse signal and a second amplifier stage which is directly (or indirectly) controlled by the output of said first amplifier stage and comprises at least one field-effect transistor.
2. Description of the Prior Art
It is a major problem in pulse generator technology to provide pulses with very fast (variable) rise and/or fall times (fast transition times). The limiting factor in most applications is the output amplifier which receives a pulse signal internally generated (internal pulse signal).
Bipolar technology is used in output amplifiers of the type described above. In such a bipolar output amplifier, a first bipolar transistor stage may receive the pulse signal internally generated. This first stage may be a differential amplifier, the outputs of said differential stage being connected to a second bipolar transistor amplifier stage. The second stage comprises bipolar transistors which are able to generate an output signal with the required output voltage swing, i.e. the necessary maximum/minimum output signal amplitude.
Usually, pulse generators have to provide an output swing of 10 v (volts). For medium output bandwidths (medium transition times of the generated output pulses), bipolar transistors are suitable.
Unfortunately, this does not apply for very fast transition times, e.g. transition times of 0.6 ns (nanoseconds) and below. Upon such fast transition times which correspond to a bandwidth of 1.0 GHz (Gigahertz), the collector-emitter breakdown voltage is in the range of 6 V and therefore not applicable for output swings of 10 V. This is one of the reasons that pulse generators with adjustable amplitudes, rise times and offsets were not yet designed to generate the very fast transition time mentioned above.
Output amplifiers based on other technologies than bipolar technology are unsuited for the application described above. For example, an output amplifier on the exclusive basis of gallium arsenide field effect transistors suffers from a very small gain. This small gain does not allow feedback loops to compensate the amplifier for temperature effects and for linearization. A further disadvantage is the so-called "droop effect", i.e. the high frequency gain is different from the low frequency gain (one should keep in mind that commercial pulse generators allow adjustment of the pulse repetition rate, i.e. the output amplifier has to be a broadband amplifier, from direct current to the maximum frequency).
It is also already known to replace the bipolar transistor in the second amplifier stage by a power MOSFET transistor, cf. U.S. Pat. No. 4,528,520 which depicts an output amplifier for a cathode ray tube. The MOSFET provides higher output swing without deteriorating the high frequency characteristic.